Exercise 1: What a Third Routine Would See — Possible Solution ==================================================================== TRACING THE SHARED STATE ------------------------------ This chapter verified that the first routine (test_a) increments the singleton counter three times, leaving count=3. The second routine (test_b) then increments it once more, leaving count=4 (verified directly in this chapter, even though test_b expected to start from 0). A third, independent routine that calls SingletonCounter() and reads count immediately, WITHOUT calling increment() at all, would see count = 4 - the exact value left behind by the second routine, since SingletonCounter() always returns the same shared object regardless of which routine calls it or how many routines called it before. WHY THE VALUE IS 4 AND NOT 0 OR 3 ------------------------------ It is not 0, because a Singleton's whole defining property (verified earlier in this chapter via x is y) is that every call returns the SAME object rather than a freshly initialized one - there is no "blank slate" waiting for a third caller. It is not 3 either, because the second routine's own increment() call already modified that same shared state after the first routine finished, so the object's count field reflects the cumulative effect of every single increment() call made by any part of the program so far, not just the most recent routine's own actions. WHY THIS IS THE KEY DANGER OF SINGLETON STATE ------------------------------ This demonstrates something even more concerning than the original two-routine example: a third routine that never even calls increment() itself can still observe a completely unexpected, non-zero value purely as a side effect of unrelated code that ran earlier - it doesn't need to actively misuse the Singleton to be affected by it, merely to read from it after something else already has. WHY THIS WORKS AS AN ANSWER ------------------------------ The answer traces the exact cumulative state left behind by both prior routines from this chapter's own verified numbers, explains precisely why a Singleton can never offer a fresh starting value to a new caller, and highlights the further implication that even a purely read-only third routine is affected by this shared state.